Holmes is asked by the country's Prime Minister to aid in the recovery of a stolen diplomatic letter, which, if published, might lead England into war.

An obscure family document containing an enigmatic ritual holds the key to a hidden treasure if Holmes can decipher the mystery.

Neville St Clair works in the City of London and is a man of very regular and respectable habits. One day, his wife goes into town for lunch and is shocked to see her husband in the window of an opium den. That evening, he fails to return home, and Mrs St Clair decides to consult Holmes.

The Duke of Holdernesse needs Holmes's help when his son, Lord Arthur Saltire, is kidnapped from the Priory School. Holmes finds the connection between an old mystery and a teacher who has also gone missing.

Holmes is curious about a series of attacks on figures of the French Emperor Napoleon Bonaparte around London. One such incident leads to a violent death, and he investigates. Lestrade sees an Italian connection and proposes to focus Scotland Yard's attention on the Italian quarter of London, but Holmes is more interested in Chiswick.